THE KORLE BU Teaching Hospital in Accra, Ghana—the third largest hospital in Africa—houses two radiation machines for treating cancer patients. Both are relatively new, purchased by Ghana’s Ministry of Health in the last few years. Both produce powerful X-rays that can penetrate your skin to kill tumor cells in your body. People from all over Ghana, even outside the country, come to the hospital to use the machines for cancer therapy.

“We have patients from Nigeria, Togo, and the Ivory Coast coming to us for treatment,” says Joel Yarney, an oncologist at the hospital.

One machine produces X-rays by accelerating electrons close to the speed of light through an intricately built copper pipe, then slamming them into a heavy metal target. It’s called a linear accelerator, or linac for short, and it's a cousin of the Higgs boson-discovering Large Hadron Collider. The other, known as an isotopic teletherapy machine, produces X-rays as silvery cobalt-60 chunks inside a small canister eject X-rays and transform into nickel through radioactive decay. Doctors then direct the X-rays toward a patient’s tumor.

The isotopic technology saves lives. But it’s also responsible for some of the most serious radiation accidents in history aside from the nuclear reactor meltdowns at Chernobyl and Fukushima.

Invented more than 60 years ago, these radioactive material-based machines soon became a fundamental tool for cancer treatment. They usually use cobalt-60 as a radioactive source, although some versions use a compound of cesium-137, cesium chloride. Around 40 years ago, American hospitals started to replace them with linacs, whose X-ray beam is much easier to shape—and today, US hospitals use them pretty much exclusively. “All else being equal, a doctor would prefer to have a linac than a cobalt machine because it’s better cancer treatment,” says Miles Pomper of the James Martin Center for Nonproliferation Studies.